For purposes of speed and econony of construction, private automatic branch exchanges for telephone networks have been constructed in which the audio data from each port is first converted to digital form for transmission through the exchange, then re-converted to analog form as supplied to another port. The digitized audio data is customarily transmitted through the exchange in serial form, with control of the data transmission through the exchange controlled by one or more microprocessors.
A digitized exchange of this kind has a number of substantial limitations and problems. Internal timing presents substantial difficulties and the serialized data transfer arrangement entails cumbersome software that is quite difficult to compose. The microprocessors impose a substantial limitation on the speed at which the transfer of data between ports can be accomplished if the exchange serves a considerble number of ports (e.g., two hundred or more). A large number of conductors are required for transmission of data between ports, in any exchange of substantial capacity, so that the full economic potential of digital operation is difficult and, in larger exchanges, virtually impossible to realize.